Implantable medical devices (IMDs) for monitoring a physiological condition or delivering a therapy typically rely on one or more sensors positioned in a patient's blood vessel, heart chamber, or other portion of the body. Examples of such IMDS include heart monitors, pacemakers, implantable cardioverter-defibrillators (ICDs), myostimulators, nerve stimulators, drug delivery devices, and other IMDs that rely on physiological signals for monitoring a patient and/or controlling a therapy. Implantable sensors used in conjunction with an IMD generally provide a signal related to a physiological condition from which a patient condition or the need for a therapy can be assessed.
As an example, measurement of mixed-venous blood oxygen saturation levels is of interest in determining the metabolic state of the patient. Generally, a decrease in mixed-venous blood oxygen saturation is associated with an increase in physical activity or may reflect insufficient cardiac output or respiratory activity. Thus monitoring mixed-venous blood oxygen saturation allows an implantable medical device to respond to a decrease in oxygen saturation, for example by pacing the heart at a higher rate.
An implantable oxygen sensor for use with an implantable medical device is generally disclosed in commonly assigned U.S. Pat. No. 6,198,952 “Multiple Lens Oxygen Sensor for Medical Electrical Lead” issued to Miesel, hereby incorporated herein by reference in its entirety. Cardiac pacemakers that respond to changes in blood oxygen saturation as measured by an optical sensor are generally disclosed in U.S. Pat. No. 4,202,339 “Cardiac Pacemaker” issued to Wirtzfeld, et al. and in U.S. Pat. No. 4,467,807 “Rate Adaptive Demand Pacemaker” issued to Bornzin, both of which patents are incorporated herein by reference in their entirety. A blood oxygen saturation measurement may also be used to augment arrhythmia detection in a pacemaker/cardioverter/defibrillator (PCD) such as set forth in commonly assigned U.S. Pat. No. 5,163,427 “Apparatus for Delivering Single and Multiple Cardioversion and Defibrillation Pulses” to Keimel and U.S. Pat. No. 5,188,105 “Apparatus and Method for Treating a Tachyarrhythmia” also to Keimel, both incorporated herein by reference in their entireties.
One limitation encountered with the use of implantable optical sensors can arise as the result of thrombus formation over the sensor windows and/or tissue encapsulation of the sensor that occurs as a result of the normal physiological response to a foreign object. Such overgrowth in the form of thrombus formation or tissue encapsulation interferes with the performance of the sensor in accurately measuring blood oxygen or other metabolites. The light reflected back to the sensor may be altered by the overgrowth depending on the optical properties of the overgrowth mass. For example, the light reflected back to the sensor may be increased due to higher reflectance of soft tissue than whole blood. Additionally, the light signal associated with blood oxygen saturation is reduced due to attenuation of emitted light from the optical sensor that reaches the blood volume and attenuation of the reflected light from the blood volume reaching a light detector included in the optical sensor.
The time course and degree of tissue encapsulation of an optical sensor, or any other implanted medical device, is uncertain. Thrombus formation in the vicinity of the sensor due to blood stasis or endothelial injury can occur at unpredictable times after device implant. Because the time course and occurrence of these events is unpredictable, the reliability of physiological measurements using an optical sensor at any point in time may be uncertain.